Artificial single crystal diamonds have potential for a variety of scientific, industrial and commercial applications, for instance, in jewelery, heat sinks, electronic devices, laser window, optical window, particle detectors and quantum computing devices. As the commercial demand for single crystal diamond increase over the years, it is essential to increase the production of high quality optical and scientific grade single crystal diamonds without compromising the quality of the single crystal diamond. Factually, the requirement of quality is very stringent on the single crystals for applications in scientific products especially for the purpose of semi-conducting devices and the particle detectors. The defects, inclusions, microscopic grain boundaries, other orientations are some prominent defects in single crystal diamonds and have to be deeply characterized in details.
The prior art so far has used one deposition chamber in which the suitable gases such as methane, hydrogen and other gases such as nitrogen, oxygen and diborane are supplied therein to grow single crystal diamonds, with the exhaust gases exiting to the atmosphere. The gases are decomposed into various ionic forms and radicals using an intense microwave electric field at a frequency of 2.45 GHz. The impurities often get incorporated in to the diamond structure from the gas lines, chambers and other sources of contaminations. A significant point to be noted, however, is that the efficiency of the decomposition of the gases in their ionic form is substantially low and it is perhaps not realized that the exhaust still may contain the constituent gases for further diamond growth. Moreover the gas composition is also purified after passing through the plasma phase as most impurities would have been removed by the plasma. It is the endeavour to understand this and utilize this fundamental fact to which the present invention is directed.
A process of growing the poly-crystalline grains of diamond was disclosed in U.S. Pat. No. 3,030,187. Since then, various chemical vapour deposition (CVD) techniques have been devised to produce poly-crystalline and mono-crystalline diamonds whereby methane and hydrogen is used as precursor gases. The role of methane is to ensure the supply of carbon in the gas phase while the hydrogen plays an important role in the stabilization of diamond phase.
Poly-crystalline diamond, despite having similar properties as mono-crystalline diamonds, is not a recommended material for new industrial applications due to the presence of grain boundaries and defects contained therein. In addition, the thermal conductivity of a poly-crystalline diamond is inferior to that of a mono crystalline diamond. Furthermore, the grain boundaries in poly crystalline diamonds play a deteriorating role and inhibits the exhibition of the superior properties unique to natural diamonds because the grain boundaries act as scattering centres. The presence of the grain boundaries in poly-crystalline diamonds are a major drawback in industrial applications.
Accordingly, there is a clear preference for using mono-crystalline diamonds in industrial applications. However, it is difficult to grow mono-crystalline diamonds with the same texture, clarity, purity and finish as those of a natural diamond. Although mono-crystalline diamond has superior properties compared to poly-crystalline diamond, microscopic and macroscopic graphitic and non-graphitic inclusions, feathers (long line defects) are very common in CVD grown mono-crystalline diamond.
Detailed characterization of defects in CVD grown mono-crystalline diamond can be performed by Raman spectroscopy and X-ray diffraction (XRD) which reveals the defects comprising of graphitic regions having a size in the range of submicrons to several microns contained therein.
The existence of the graphitic and non-graphitic inclusions in the mono-crystalline chemical vapour deposited diamond (CVD diamond) may be due to the presence of un-reacted methane in the deposition chamber. Almost all techniques employ a mixture of methane and hydrogen gases for the CVD of diamond. The methane gas is electrically decomposed leading to the formation of excited methyl group species (CH3+ ions) due to the electric field of microwaves of 2.45 GHz frequency. The electrical discharge of the methane and hydrogen gases form a hot plasma consisting of CH3+ ions, atomic hydrogen, H2+ ions and a significant concentration of electrons. The plasma region of the prior art is of substantially ellipsoid shape and it engulfs the substrate stage assembly completely.
Prior art substrate stages are generally made of molybdenum in the shape of a flat disc which is used as a pedestal for loading the diamond seeds (substrates) of the sizes varying from 1 mm×1 mm to 10 mm×10 mm and having a thickness of 1 mm to 3 mm as the case may be. The pedestal can also be made of tungsten or any other suitable metal. As the methyl ions reach substrate at a temperature 900° C., their mobility is high and they start forming a sp3 bonded diamond network in presence of high concentration of hydrogen. The boundary (outer periphery) of the plasma region may contain the neutral molecular methane gas and it may decompose thermally. The thermal decomposition of the methane occurs at 800° C. and the result of the thermal decomposition is the formation of black carbon soot that can induce the graphitic and non-graphitic impurities in the diamond deposit.
It is an objective of the present invention to provide a substrate stage which provides uniformity of microwave electric field and increase the concentration of CH3+ ions in the plasma region and reduces the ratio of un-reacted methane in the plasma region. The substrate stage also ensures that the heat current flows in such a way so that the temperature of the periphery of the stage is much lower than the rest of the pedestal. As a result, the carbon soot formation can be entirely avoided.